A vehicle drive train with two drivable vehicle axles is known from WO 2009/127324 A2. The vehicle drive train comprises a transmission, coupled with a drive motor and/or a drive unit and designed as a gear wheel shift transmission, with several shiftable transmission stages, along with a first differential gear coupled with the gear wheel shift transmission, which is coupled with the wheels of a first drivable vehicle axle through two first drive shafts. The first drivable vehicle axle is constantly driven, and represents a driven front vehicle axle. Furthermore, a transfer gearbox and/or transfer device coupled with the first differential gear is provided, which is coupled through a connection shaft with a second differential gear, which is in turn in operative connection with the wheels of a second drivable vehicle axle through additional drive shafts. The operative connection between the first differential gear and the transfer gearbox can be separated at the wheels of the two drivable vehicle axles by means of a shiftable clutch for the interruption of a transfer fo the driving force and the turning moment. This shiftable clutch is arranged between the first differential gear and the transfer gearbox.
Depending on the operating state, uncoupling the part of the vehicle drive train leading in the direction of the second drivable vehicle axle uncouples in the immediate vicinity of the gear wheel shift transmission through the actuation of the shiftable clutch. In order to minimize the friction losses of the partial drive train that is uncoupled by means of the shifting clutch and, in this shifting position, is not in operative connection with the rest of the drive train, additional clutches are provided in the area of the drive shafts of the two drivable vehicle axles. In the open operating state of the additional clutches, the non-driven partial drive train between the transfer gearbox and the additional clutches comes to a standstill.
Disadvantageously, when the vehicle drive train is operated during an open operating state of the two additional clutches between the halves of the shifting element of the additional clutches designed as friction-locking clutches, there are differential rotational speeds that cause unwanted drag torques and impair the efficiency of the vehicle drive train and increase the fuel consumption of a drive motor designed as an internal combustion engine.
Furthermore, a four-wheel drive system with a drive unit, a main transmission for displaying different transmission ratios along with a differential gear provided in the area of a first drivable vehicle axle for distributing the drive force between the two drive wheels is known from DE 40 39 392 A1. A second drivable vehicle axle is able to be brought into operative connection with the drive unit through a decoupler. The turning moment (torque) value that is able to be led through the decoupler in the direction of the second drivable vehicle axle is diverted through a bevel gearbox in a vehicle transverse direction and is able to be led through wheel couplings in the direction of the drive wheels of the second drivable vehicle axle.
Depending on the respective operating status of the four-wheel-drive system, the decoupler for decoupling the second drivable vehicle axle is opened. In order to reduce friction losses in the area of the bevel gearbox, wheel couplings that are additionally arranged in the area between the bevel gearbox and the drive wheels of the second drivable vehicle axle are likewise opened. In turn, differential rotational speeds present between the halves of the shifting element of the opened friction-locking wheel couplings have drag losses.
For the further reduction of the power losses, positive-locking shifting elements instead of friction-locking shifting elements are provided in the area of vehicle drive trains, through which the side shaft separator known from the state of the art is possible in the area of a drivable vehicle axle. Moreover, with such vehicle drive trains, the supporting connection in the open operating state of the positive-locking shifting element is separated between a differential shaft of a differential gear and the associated wheel. Thus, the shutdown of a so-called “cardan drive” with the accompanying standstill of a cardan shaft and/or a shaft connected to a transfer box with the differential gear, a set of bevel wheels on the differential gear along with a crown wheel carrier and/or a differential carrier is enabled.
At vehicle speeds greater than zero, in the coupled condition and/or upon a rotational speed of a crown wheel and of the differential carrier connected with it of essentially zero, there is a rotational speed compensation in the area of the differential gear. During the rotational speed compensation, the two halves of the shifting element of the open positive-locking shifting element in the area between the differential shaft and an output shaft leading in the direction of the wheel rotates with the wheel rotational speed, whereas, the halves of the shifting element circulate with the opposing direction of rotation. Thereby, during a driving operation, a control sleeve of the positive-locking shifting element coupled to one of the shafts always undergoes a rotational movement. This leads to the fact that, for the actuation of the control sleeve between an open operating state of the positive-locking shifting element and a closed operating state of the positive-locking shifting element, stationary actuator devices are exposed to a high degree of mechanical wear based on the permanently present differential rotational speed between the rotating control sleeve and the actuator device.
In order to design a shiftable four-wheel drive of a vehicle with a desired long service life, a correspondingly high structural expenditure is to therefore be provided in the area of one or more shiftable shifting elements and the actuator devices allocated to each of them, yet, due to reasons of installation space and costs, this is not desired.